When optical cables are installed in aerial, they are exposed to constant tensions and, as a result, they may be subject to undesirable elongations. The tensile forces applied to aerial optical cables depend in particular on the climatic elements (wind, ice, etc.), the physical specifications (length, weight, etc.) of the cables, and how the cables are installed on the field.
To limit elongation that may occur in aerial optical cables, such cables are usually made as small and light as possible. Lightweight and small cables can be produced using a micromodule cable design. In aerial micromodule cables, optical fibers are grouped to form one or plural micromodules gathered as a cable.
In aerial micromodule cables, and more generally in micromodule cables, the micromodule sheaths are thin and flexible, this resulting in a low resistance of these cables to external forces. In consequence, the cable jacket cannot be extruded tight over the cable core as this would cause excessive stress on the optical fibers, thereby increasing attenuation. In micromodule cables, the optical core formed by the optical fibers is therefore uncoupled from the cable jacket.
Additionally, the cable core is generally designed so as to exhibit some level of excess length relative to the length of the cable jacket (typically about 0.2%). Having such an excess fiber length (EFL) in the cable ensures that the optical fibers are not elongated unless the cable is loaded above a certain level of tension.
Conventional techniques are known to achieve a certain level of excess fiber length in micromodule cables.
Excess fiber length can be controlled to some extent by adjusting geometrical construction parameters of the cable jacket and optical core.
Document U.S. Pat. No. 5,125,063 (AT&T Bell Laboratories) discloses an optical fiber cable suited for aerial distribution use, for example, including in a preferred embodiment at least one bundle of optical fibers. The at least one bundle is disposed in a tubular member which is made of a plastic material suitable for use in a relatively wide temperature range and which is enclosed by a sheath system. A predetermined excess length of fiber is caused to be disposed in the tubular member. The excess length of each fiber is such that it is sufficient to avoid undue strains on the fiber as the cable core is exposed to the elements and to forces imparted during handling such as during installation. This document makes reference to a manufacturing technique described in document U.S. Pat. No. 4,446,686 (AT&T Bell Laboratories) to obtain core excess length.
More particularly, document U.S. Pat. No. 4,446,686 discloses that, in the manufacture of a lightguide fiber cable in which a lightguide fiber core is loosely disposed in a composite sheath, it is important to control the ratio of the lengths of the core and sheath. A core which is shorter than the sheath and which follows a shortened path on a reel may be unduly strained when the cable is installed in the field. This problem is overcome by coupling the core to the sheath by a system which includes a constant speed linear capstan and a relatively large variable speed sheave that is positioned between the linear capstan and a takeup reel. The coupling of the core to the sheath is accomplished on the sheave after the sheath is elongated between the linear capstan and the sheave. The coupling and the elongation cooperate to compensate for the inherent shortfall in core length which otherwise would occur when the cable is wound on a reel. As a result, when tension forces that have been applied to the sheath are reduced, the sheath, which includes strength members having relatively high modulus of elasticity, recovers to its original length and the ratio of the length of the core to that of the sheath becomes a predetermined value.
Document U.S. Pat. No. 5,372,757 discloses that a loose tube element including a plastic sheath containing a grease-like material and an optical fiber or bundle of optical fibers or a ribbon of optical fibers is pulled from an extruder crosshead through a cooling bath and around a constant speed capstan. The loose tube element travels around the constant speed capstan a number of times and as it exits the capstan it is subject to a tensile force provided by a variable torque control capstan. The tensile force causes elongation of the plastic sheath, causes the optical fiber to be pulled taut and thereby controls the ratio of fiber length to sheath length within the loose tube element. While subjected to the tensile force, the element is subsequently cooled, causing the elongation of the plastic sheath to become permanent and thus fixing the fiber-to-sheath length ratio. The fiber-to-sheath length ratio is not affected by variation in production line speed and the production line may operate at very high speeds while producing loose tube elements with consistent fiber-to-sheath length ratios. The loose tube element is then stored on a take-up reel or is used as a stranding element or core element in a fiber optic cable.
However, only a limited amount of excess fiber length can be achieved using a conventional cable manufacturing method such as one of those mentioned above. Further, employing for instance the manufacturing method of document U.S. Pat. No. 4,446,686 can be problematic since it generally requires applying important tensions to the cable jacket. It is not always practical or feasible on a production line to apply the required level of tensile forces to the sheath.
No conventional cable manufacturing method allows producing in an efficient and reliable manner aerial optical cables, more particularly aerial micromodule cables, with a sufficiently high excess fiber length so as to limit or avoid undesirable tensions in these cables during or after their installation on the operative field.